The data on year structure built were obtained from Housing Question 2 in the 2006 American Community Survey. The question was asked at both occupied and vacant housing units. Year structure built refers to when the building was first constructed, not when it was remodeled, added to, or converted. Housing units under construction are included as vacant housing if they meet the housing unit definition, that is, all exterior windows, doors, and final usable floors are in place. For mobile homes, houseboats, RVs, etc., the manufacturer's model year was assumed to be the year built. The data relate to the number of units built during the specified periods that were still in existence at the time of interview.

Median year structure built divides the distribution into two equal parts: one-half of the cases falling below the median year structure built and one-half above the median. Median year structure built is computed on the basis of a standard distribution (See the "Standard Distributions" section under "Derived Measures.") The median is rounded to the nearest calendar year. Median age of housing can be obtained by subtracting median year structure built from survey year. For example, if the median year structure built is 1968, the median age of housing in that area is 40 years (2006 minus 1968). (For more information on medians, see "Derived Measures.")

Data on year structure built are more susceptible to errors of response and non-reporting than data for many other questions because respondents must rely on their memory or on estimates by people who have lived in the neighborhood a long time.

The 1996-1998 American Community Survey question provided a write-in space for the respondent to enter a year. Since 1999, the question provided 9 pre-coded response categories which showed ranges of years. The response categories were updated starting in 2003 to provide detail for recently built structures.